In order to support a higher data transmission rate and to provide users with high-quality service, multi-carrier technologies are supported at present in both a Universal Mobile Telecommunication System (UMTS) and a Long Term Evolution-Advance (LTE-A) system, that is, resources of a plurality of carriers are aggregated to obtain a lager bandwidth and to serve a terminal together.
1) Multi-Carrier/Multi-Cell Technologies in UMTS
In order to improve a user peak rate and cell data throughput, in the UMTS system, multi-cell technologies are introduced for a Frequency Division Duplexing (FDD) mode, while multi-carrier technologies is introduced for a Time Division Duplexing (TDD) mode. The multi-carrier/multi-cell technologies are supported at present for both a High Speed Downlink Packet Access (HSDPA) and a High Speed Uplink Packet Access (HSUPA).
For the FDD mode, there is only one carrier in a cell, and the multi-cell technologies refer to aggregation of a plurality of consecutive or inconsecutive carriers under a same NodeB together to serve a UE concurrently to thereby provide a desired rate. The multi-cell technologies involve dual cells in the downlink, four cells in the downlink, dual cells in the uplink, etc., each cell of which is a backward-compatible cell that can independently operate, and when they serve a terminal concurrently, there is one and only one primary cell, and the others are all secondary cells.
For the TDD mode, there are multiple carriers in a TDD cell, and the multi-carrier technologies refer to aggregation of multiple carriers of the same TDD cell for communication by a UE. Multiple carriers in the TDD mode include three carriers in the downlink, six carriers in the downlink, and three carriers in the uplink, etc., and all of these carriers that can operate concurrently belong to the same TDD cell; and for the UE, only a primary cell is a backward-compatible cell that can independently operate, and the other secondary cells can be regarded as resources for use only under the multi-carrier technologies.
2) Carrier Aggregation Technology in LTE-A
In an LTE system, there is only one carrier with the maximum bandwidth of 20 MHz in a cell, as illustrated in FIG. 1.
In the LTE-A system, peak rates of the system have been greatly improved over the LTE system, and are required to reach 1 Gbps in the downlink and 500 Mbps in the uplink. The required peak rates can not be reached if only one carrier with the maximum bandwidth of 20 MHz is used. Thus, the LTE-A system has to extend the bandwidth available to the terminal, and to this end, a Carrier Aggregation (CA) technology has been introduced, that is, a plurality of consecutive or inconsecutive carriers under a same evolved NodeB, eNB, are aggregated together to serve the UE concurrently to thereby provide a desired rate. These carriers aggregated together are also referred to Component Carriers (CCs). Each cell can be a component carrier, and cells (component carriers) under different eNBs can not be aggregated.
In order to ensure that the UE in the LTE system can operate over each aggregated carrier, the bandwidth of each carrier shall not exceed 20 MHz at most. The CA technology of the LTE-A system is as illustrated in FIG. 2, and under the evolved NodeB in the LTE-A system illustrated in FIG. 2, there are four carriers that can be aggregated, and the evolved NodeB can transmit data with the UE concurrently over the four carriers to thereby improve system throughout.
An inter-system interoperation technology, that is, Inter-Radio Access Technology (Inter-RAT), refers to a technology of interoperation between different systems, e.g., a handover technology between the UMTS system and the LTE system (Packet Switched Handover (PS HO)) and a redirection technology, and the use of these technologies can enable cooperative operation between different heterogeneous networks to better serve a multimode terminal. For a multimode terminal supporting both the UMTS system and the LTE system, a single-carrier handover between them is supported in existing protocols in order to ensure service continuity thereof and will be detailed below.
As illustrated in FIG. 3, it is an architecture of a UMTS network, which includes two parts: a Core Network (CN) and a UMTS Terrestrial Radio Access Network (UTRAN), wherein access network nodes in a PS domain includes a NodeB and a Radio Network Controller (RNC) which is connected with a Serving GPRS Support Node (SGSN) and a Mobile Switching Center (MSC)/Visitor Location Register (VLR). As illustrated in FIG. 4, it is an architecture of an LTE (also referred to as E-UTRAN) network, wherein access network nodes include an evolved NodeB, eNB, connected with a Mobility Management Entity (MME)/Serving Gateway (S-GW).
As illustrated in FIG. 5, it is an Inter-RAT network architecture between UMTS and LTE. The existing UMTS device, SGSN, has to be updated to support an interface S4 and thus can be referred to as an S4 SGSN. In this network architecture, all the user plane data passes through two core network nodes which are a Packet Data Network Gateway (PDN GW) and the Serving GW. The SGSN and the MME transmit control plane signaling via an interface S3.
When a terminal resides in the UMTS network, the terminal receives user plane data through the PDN GW, the Serving GW, the SGSN, the RNC and the NodeB via interfaces S5, S4, Iu and Iub; and when the terminal switches to the LTE network, the terminal receives user plane data through the PDN GW, the Serving GW and the eNB via interfaces S5 and S1-U.
If the terminal is intended to forward data, then there are two schemes, in one of which a direct data forward tunnel is established between the RNC and the eNB through the SGSN and the MME; and in the other of which an indirect data forward tunnel is established between the RNC and the eNB, and the data of the terminal is forwarded to the eNB through the RNC, the SGSN and the Serving GW, or a direct tunnel is established by the SGSN between the RNC and the Serving GW, and the data of the terminal is forwarded to the eNB through the RNC and the Serving GW.
The terminal may switch between different RATs for the reason of movement or channel quality. In order to assist the network to perform a more reasonable handover decision, the UE will measure and report channel quality of another RAT depending on network configuration. There are different measurement parameter configurations and separate measurement procedures for inter-RAT measurement and inter-system measurement, where a measurement gap is typically used. For example, when the E-UTRAN system measures the UTRAN system, an event B1 or an event B2 can be configured and a measurement gap can be configured depending on the capability of the UE. Specific contents of the event B1 and the event B2 are as follows:
Event B1: channel quality of an adjacent cell of a disparate system is above a threshold; and
Event B2: channel quality of a serving cell is below a first threshold, and channel quality of an adjacent cell of a disparate system is above a second threshold.
When the UTRAN system measures the E-UTRAN system, events 3a, 3b, 3c and 3d can be configured particularly as follows:
Event 3a: channel quality of a serving cell is below a first threshold, and channel quality of an adjacent cell of a disparate system is above a second threshold;
Event 3b: channel quality of an adjacent cell of a disparate system is below a threshold;
Event 3c: channel quality of an adjacent cell of a disparate system is above a threshold; and
Event 3d: the strongest cell in an adjacent cell of a disparate system is changed.
Based upon the foregoing network architecture and Inter-RAT measurement mechanism, a simplified flow chart of switching by a terminal from UMTS to LTE at the Radio Access Network (RAN) side is as illustrated in FIG. 6 and generally includes the following steps:
A source RNC sends a measurement configuration message to the terminal; the terminal measures according to the measurement configuration message and reports a measurement result to the source RNC; the source RNC makes a handover decision in view of the reported measurement result; the source RNC sends a handover request to a target eNB through the core network when deciding to switch to LTE; the target eNB sends a handover request acknowledgement to the source RNC through the core network according to the handover request; the source RNC sends a handover command to the terminal; the terminal accesses the target eNB upon reception of the handover command; and the terminal sends a handover completion message after successfully accessing the target eNB.
Particularly, all of the measurement configuration message, the measurement report message, the handover command and the handover completion message are signaling of the access network side. Both the handover request transmitted from the source RNC to the target eNB and the handover request acknowledgement transmitted from the target eNB to the source RNC are containers in interface messages in conformity with a message encapsulation format of the core network and transmitted through the core network. The handover request acknowledgement message contains an RRC container with contents thereof being the handover command, and the source RNC transmits this message to the terminal via a null interface.
Based upon the foregoing network architecture and Inter-RAT measurement mechanism, a simplified flow chart of switching from the LTE system to UMTS at the RAN side is as illustrated in FIG. 7 and generally includes the following steps:
A source eNB sends a measurement configuration message to a terminal; the terminal measures according to the measurement configuration message and reports a measurement result to the source eNB; the source eNB makes a handover decision in view of the reported measurement result; the source eNB sends a handover request to a target RNC through the core network when deciding to switch to LTE; the target RNC sends a handover request acknowledgement to the source eNB through the core network according to the handover request; the source eNB sends a handover command to the terminal; the terminal accesses the target RNC upon reception of the handover command; and the terminal sends a handover completion message after successfully accessing the target RNC.
Particularly, all of the measurement configuration message, the measurement report message, the handover command and the handover completion message are signaling of the access network side. Both the handover request transmitted from the source eNB to the target RNC and the handover request acknowledgement transmitted from the target RNC to the source eNB are containers in interface messages in conformity with a message encapsulation format of the core network and transmitted through the core network. The handover request acknowledgement message contains an RRC container with contents thereof being the handover command, and the source eNB transmits this message to the terminal via a null interface.
No matter whether the handover is from UTMS to LTE or from LTE to UMTS, the measurement configuration message sent from the source access node includes measurement configuration contents, e.g., a measurement object, configured events, a measurement result reporting scheme and the like. The terminal measures according to the measurement configuration contents and reports the measurement result in the configured measurement reporting scheme.
Taking the handover from UTMS to LTE as an example, based upon existing protocols, Inter-RAT measurement in the measurement configuration message sent from the source RNC includes the measurement configuration contents. E-UTRAN Measured Results in the measurement report message reported by the terminal include the measurement result, and E-UTRAN Event Results in the measurement report message include an event result.
Contents of an Information Element (IE) of the E-UTRAN Measured Results particularly include: a carrier frequency of E-UTRAN, and an identifier and a measurement result of a cell satisfying a report condition at the frequency. The measurement result is typically represented by Reference Signal Receiving Power (RSRP) and Reference Signal Received Quality (RSRQ).
Contents of an Information Element (IE) of the E-UTRAN Event Results particularly include: an Inter-RAT measurement event ID, a carrier frequency of E-UTRAN, and an identifier of a cell satisfying a report condition at the frequency.
In the Inter-RAT measurement mechanism in the prior art, the measurement object configured for measurement is a single frequency, that is, only one frequency or a cell at the single frequency is configured, so only an Inter-RAT single-carrier handover but no multi-carrier handover is supported. At present, the multi-carrier technologies have been introduced to both the UMTS and LTE systems to improve the data transmission rate of the user, but when the user using the multi-carrier technologies switches between these two systems, he has to firstly fall back to the single-carrier state to perform a handover of the PS domain and can enter the multi-carrier state again only after finishing the handover. This will undoubtedly lower the data transmission rate of the user and degrade user experience. Moreover, when the terminal switches from another system supporting high-speed data transmission to UMTS or LTE, the multi-carrier handover to these two systems is not supported, thus also lowering the data transmission rate of the user and degrading user experience.